The present invention relates to an imaging method using nuclear magnetic resonance (which will be shortly referred to as "NMR") and, more particularly, to an imaging apparatus for imaging a nuclear density distribution or a relaxation time distribution by measuring the nuclear magnetic resonance signal of hydrogen or phosphor in an organism and an imaging method therefor.
In the prior art, an X-ray CT or an ultrasonic imaging apparatus is widely used as an apparatus for imaging in vivo the internal structure such as the head or abdomen of a human body. In recent years, it has been revealed that the trial to make similar images by using the nuclear magnetic resonance phenomenon has succeeded so that the data which cannot be attained by the X-ray CT or the ultrasonic imaging apparatus can be achieved. In the imaging apparatus using the nuclear magnetic resonance, it is necessary to separate and discriminate the signal from an object to be imaged in a manner to correspond to the individual portions of the object. There is one method for obtaining positional data of an object to be detected by applying a gradient field to the object to make different the static fields, in which the portions of the object are placed, thereby to make different the resonance frequencies of the individual portions. For example, in an imaging method called the "projection-reconstruction method", which is disclosed by C-M La and P. C. Lauterber, Journal of Physics (E) Scientific Instrument Vol. 13, 1980, pp 747-750, the direction of the gradient field is turned step by step, and the projection data of the resonance signals, which are obtained as functions of the intensities of the magnetic field for the respective angles of turns so that the images indicating the nuclear spin distribution of the object may be obtained.
As is disclosed by J. M. S. Hutchison, et al., Journal of Physics (E) Scientific Instrument Vol. 13, 1980 pp 947-955, the difference in longitudinal relaxation time at the individual portions of the object appears in a superposed manner on the spin intensity distribution if RF magnetic pulses for rotating the nuclear spin 180 degrees are applied prior to the measurement of the individual projection data and if the nuclear spin is excited again after a predetermined time has elapsed. This image will be called a "relaxation time enhanced image" in the following.
The distribution of the relaxation time may frequently become more important data than the distribution of the spin intensity. However, the absolute value of the relaxation time cannot be known from a single relaxation time enhanced image but can be determined either from both an intensity image indicating the intensity distribution of the nuclear spin and the relaxation time enhanced image or from a plurality of relaxation time enhanced images having different relaxation time effects. Despite of this fact, the imaging apparatus of the prior art has measured the intensity images and the relaxation time images, or the plural relaxation time enhanced images of different relaxation time effects independently of one another. As a result, the measurement time for obtaining those plural images has been the summation of the measurement times for obtaining the individual images.